Romo is 36. QBs of his caliber generally play at least thru their age 38 season. So three seasons of quality play left maybe more. I wouldn't count on more than 3 years. I'd like to see his replacement have two years of internship. You can argue we waited too long already and Romo will miss time this year and next. That is a false argument as any NFL player can suffer a season ending injury at any time; it is a risky job. You should play the odds. And you really can't carry an equal backup given the salary cap anymore.
The problem with the argument of drafting the BPA at all times is it doesn't reflect the needs of the team. You can argue just get quality players each pick and eventually you will fill the roster. It doesn't work that way due to injuries, deaths, incarcerations, early retirements, suspensions and most importantly the salary cap and contracts. You don't own the rights to a player for their entire career anymore. You have them the length of their practical contract life.
So the Cowboys enjoy a view from the top this year which is unusual for them. If they want the opportunity to draft a QB who poses a good risk at being elite then they must draft one this year.
There are other QBs who I think have a chance to develop who can be drafted later but that is a risk, too.